(1) Field of the Invention
This invention relates to recycling of fluids which have been polluted or contaminated. The majority of the fluids treated according to this invention are used in natural gas treating plants, primarily Triethylene Glycol (also called TEG). In addition, ethylene glycol and diethylene glycol, which find use as anti-freeze, also accumulate hazardous material and are recycled by this process.
Operators of natural gas dehydration and treatment plants will have ordinary skill in this art.
(2) Description of the Related Art
Natural gas flowing from wells may contain water vapor, liquid water, brine solution, heavy and light hydrocarbons, and particulate matter such as sand, pipeline scale and rust. If these elements remain in the gas stream they will cause numerous problems in the pipeline and processing equipment. Treating natural gas will commonly occur at individual wells, gathering systems, compressor stations, distribution stations, and gas processing units. Lowering the water content of natural gas at these sites will prevent the clogging of pipelines due to hydrate formation. Hydrate formation will occur with the combining of water and natural gas molecules. Hydrates will block valves and flowmeters. Hydrates will also accumulate at low points in the pipeline. Hydrates will decrease pipeline efficiency and cause shutdowns. The hydrates will also increase erosion and corrosion. To prevent the formation of hydrates, natural gas must be dehydrated at the aforementioned sites prior to reaching the pipeline.
Processes for removal of entrained water vapor and other contaminants in natural gas are well known. The most common process for the removal of water in natural gas is glycol dehydration. The process is done in a natural gas dehydration plant. Where TEG is the preeminent desiccant used in the dehydration process. TEG offers the following advantages: 1) ability to absorb large amounts of water, 2) relatively low solubility of valuable gas constituents, 3) chemical stability, 4) easy to regenerate, and 5) low cost. Before treating natural gas with glycol dehydration, the wet natural gas passes through an inlet separator where water droplets, liquid hydrocarbons, entrained sand, rust and so forth are separated out of the gas stream. The wet natural gas then flows to the absorber or contacting tower where it is interfaced with the TEG. The wet natural gas will enter the contacting tower at the bottom where it will flow upward, while lean TEG, free of water, will enter the top of contacting tower where the TEG flows downward. The counter current flow will aid the lean TEG in absorbing most of the water contained in the natural gas. The natural gas stream leaving the tower is said to be dehydrated. The TEG leaving the tower is called rich TEG. The rich TEG is passed through sock filters to remove any particulate matter picked up by the TEG. The rich TEG then flows to a reboiler or regenerator where it is heated to drive off any of the absorbed water contained in the rich TEG solution. After the TEG has been regenerated, it is then recycled for reuse in the dehydration system. The TEG is recirculated numerous times per hour through the entire dehydration system(absorber tower and regenerators).
In normal use a supply of TEG will run for several months before it gets so laden with impurities that it is no longer efficient to continue use. Many of the contaminants are hazardous materials requiring expensive limitations upon their disposal. This refining unit is preferably mounted on a trailer so that it may be moved to a dehydration system where it is needed to refine or purify the spent TEG and return it to storage for reuse. The TEG can also be used to clean the dehydrator. Those in the art will understand that the dehydrators become loaded with contaminants which are hazardous, which in turn, requires expensive disposal. However, with this process, the TEG can be circulated through the dehydrator system and greatly reduce the volume of material requiring disposal.
For background information relating to glycol dehydration systems for treating natural gas, reference may be had to U.S. Pat. Nos. 5,163,981; 5,116,393; 4,375,977 and 4,661,130.
The problems encountered with TEG dehydrators is that along with water, the TEG starts to pick up small amounts of light liquid hydrocarbons. These hydrocarbons are not as easily removed as the water in the regeneration phase and a certain amount of the hydrocarbons remain with the lean TEG as it circulates back through the absorber column. The hydrocarbons attract other contaminants along with more hydrocarbons that are found in the gas stream and the TEG becomes further diluted with pollutants. As the TEG becomes more saturated, contaminants and hydrocarbons are increasingly more difficult to remove in the regeneration process. Some aromatic hydrocarbons are passed along with the water vapor into the atmosphere. These aromatic hydrocarbons are considered pollutants. They include benzene, toluene, ethylene and xylene, commonly known as BTEX. They are environmentally hazardous and considered carcinogens. These and other hydrocarbons that may be generated in the process of dehydrating natural gas are referred to as volatile organic compounds (VOC). The control of BTEX and other VOC emissions from TEG dehydration units is of increasing concern to environmental protection both at the federal and state levels. Air quality regulations in the United States are increasing because of the Clean Air Act Amendments (CAAA) of 1990. Other regulations include the National Emission Standard Hazardous Air Pollutants (NESHAP) program and state regulatory agencies.
Another source of contamination to the TEG system is salts. Carry over of brine solutions from the field can lead to salt contamination in the TEG system. Sodium salts (typically sodium chloride) are a source of problems in the reboiler since sodium chloride is less soluble in hot TEG than in cool TEG. Salts will precipitate from the solution at typical reboiler temperatures of 350 to 400 degrees Fahrenheit at atmospheric pressure. The salt can deposit on the fire tube restricting heat transfer, causing the temperature of the fire tube increase, which will lead to thermal degradation of the TEG. The salt will also increase corrosion of the fire tube. The dissolved salts cannot be removed by mechanical filtration. When the salt content reaches 1% the TEG is spent and should be reclaimed or replaced.
After a period of time, the TEG becomes severely contaminated and loses its effectiveness as a desiccant and is considered "spent". TEG at 94% concentration in solution becomes increasingly ineffective as a desiccant. The presence of contaminants may result in fouled equipment, foaming, poor dehydration and the potential of increased release of pollutants into the atmosphere. The options for spent TEG are to dispose of it and replace it with new TEG. The spent TEG may also be sent to a reclaimer for recovery. (Both of which are not very economic and will require the dehydration system to be shutdown.) During these down times operators currently choose to clean the dehydration system at considerable costs, which produces large amounts of hazardous waste to be disposed of. This also increases the chance of spilling the hazardous waste onto the ground causing more problems.